Some Conventions

The component origin P0 is generally at the intersection of P1 and P2.

P1 is normally the arrive point, P2 is the leave point and P3 is any third connection point.

For straight components P0 is normally at the arrive point, however for the likes of dampers, valves or grilles P0 will be wherever sensible, may be central or even at P2.

The axes are shown on the sketch above as used on HVAC components where Z is opposite to the PArrive, X and Y are the respective width and height of the arrive duct size.

On angled components PLeave is primarily towards X rather than -X.